Polylactide Fibers

ABSTRACT

Polylactide fibers are made from a blend of polylactides. One of the polylactides has a ratio of R-lactic and S-lactic units from 8:92 to 92:8. The second polylactide has a ratio of the R-lactic and S-lactic units of ≧97:3 or ≦3:97. The ratio of the R-lactic units to S-lactic units in the blend is from 7:93 to 25:75 or from 75:25 to 93:7. The polylactide fiber contains at least 5 Joules of polylactide crystallites per gram of polylactide resin in the fiber.

This invention relates to polylactide fibers.

Polylactide (also known as polylactic acid or “PLA”) is a thermoplasticpolymer that is useful in a variety of applications. Among these is theproduction of various types of fiber products.

Some of the uses for polylactide fibers take advantage of the ability ofthese fibers to degrade under certain conditions. Unlike many otherpolymeric fibers, polylactides can degrade rapidly under the properconditions, and in doing so form lactic acid or lactic acid oligomersthat can be consumed by biological organisms and which are soluble inaqueous environments. Therefore, polylactide fibers have potential usesin the agricultural, forestry, marine and oil/natural gas industries.For example, polylactide fiber sheet products have been proposed for useas plant coverings, to shield young plants from direct sunlight. Theseplant coverings ideally can degrade in place, so they do not have to becollected when no longer needed and then stored and/or disposed of.Instead, it is desired that the coverings degrade into the soil, wherethe degradation products can be consumed by microbes.

In the oil and gas industry, polylactide resins are used in subterraneanapplications. See, e.g., U.S. Pat. Nos. 6,949,491 and 7,267,170. Theirutility is based upon their capacity to degrade under conditions oftemperature and moisture that exist in the well. For example,polylactide resins are sometimes used in hydraulic fracturingoperations. In hydraulic fracturing, a fluid is pumped down the well andinto the surrounding formation under high pressure. This creates orenlarges fissures in the formation and so provides pathways for gas andoil to flow to the well bore. The fracturing fluid contains aparticulate solid, called a proppant, which is carried into the fissuresand prevents the fissures from closing back up once the pressure isremoved. One function of the polylactide is to help suspend the proppantin the fracturing fluid and carry it down the well bore and into theformation. The polylactide fibers are then deposited with the proppantin the fissures. The polylactide fibers then dissolve, leaving“wormholes” through with gas and oil can flow into the well.

Another use for polylactide resins in subterranean applications is theproduction of porous cements. Porous cements are sometimes wanted aswell casings and gravel packs, again for the purpose of allowingproduction fluids to pass through and enter the well. One way ofaccomplishing this is to include particles of an acid-soluble carbonatecompound in the cement composition. A polylactide resin can be includedin the cement composition. The resin becomes trapped in the cement as ithardens and then degrades to produce an acid that dissolves thecarbonate compound and produce the desired pores.

The rate at which the polylactide degrades is important in uses such asthe agricultural and subterranean applications mentioned above. Thedegradation of polylactide is believed to proceed mainly throughhydrolysis. The degradation rate is highly dependent on localconditions, including the temperature. Although polylactide oftendegrades rapidly when the local temperature is above 80° C., these veryhigh temperatures are not present in agricultural areas, nor are theypresent in many subterranean formations. In those cases the polylactidecan degrade quite slowly. Therefore, there is a desire to providepolylactide that degrades rapidly under more moderate temperatureconditions.

As mentioned in both U.S. Pat. Nos. 6,949,491 and 7,267,170, thecrystallinity of a polylactide can affect its degradation rate. Forexample, U.S. Pat. No. 7,267,170 mentions that poly-L-lactide is acrystalline polymer that hydrolyzes slowly. As such, it is suitable onlyif slow degradation can be tolerated. Conversely, U.S. Pat. No.7,267,170 mentions that poly (D,L-lactide) is amorphous and degradesmore rapidly, and suggests that this polymer may be suitable in somecases. U.S. Pat. No. 6,949,491 reports that a copolymer made from 13%D-isomer and 87% L-isomer degrades over several hours in boiling waterto form a viscous liquid. However, a polymer containing only 6% of theD-isomer was reported not to degrade under those conditions, and forthat reason U.S. Pat. No. 6,949,491 concludes “the relative amount of D-and L-isomer should be selected in the range from about 10 percent to 90percent of an isomer”. Polymers such as those are unable to crystallizeexcept at most to a very small extent. In effect, U.S. Pat. No.6,949,491 suggests to use a highly amorphous polylactide fiber.

Unfortunately, practical problems prevent highly amorphous polylactidefiber from being used in these applications. These problems have to dowith fiber production in the first instance, and with transportation andstorage in the second instance.

Highly amorphous polylactide polymers have been nearly impossible toproduce at commercially acceptable operating rates. Fiber productionprocesses require the resin to be processed at elevated temperatures,first to melt the resin and form it into fiber, and then while thenewly-formed fiber undergoes subsequent processing steps, such as, forexample, drawing and heat-setting. The problem with highly amorphouspolylactide polymers is that they are tacky at the operationaltemperatures. Therefore, they stick to the equipment, which leads to ahost of problems, including fiber breakage, frequent line stoppages, theindividual filaments sticking together to form a hard mass, productinconsistencies, and so on. These problems can be overcome by operatingat low temperatures and low manufacturing speeds, but fiber cannot beproduced economically in large volumes under those conditions. Inaddition, the fibers cannot be “heat set”, and so are very dimensionallyunstable and exhibit large amounts of shrinkage, which precludes theiruse in almost every application.

The second problem with highly amorphous polylactide resin fibers isthat they have a strong tendency to stick together and form large masseswhen they are stored. This can occur at temperatures as low as roomtemperature, but is mainly a problem when the fibers are exposed tomoderately elevated temperatures, such as from 30-50° C. Exposure totemperatures such as these is very commonly seen in warehouses andduring transportation. Therefore, special storage and handlingconditions are necessary.

These problems with making and storing amorphous polylactide fibers areso troublesome that amorphous polylactide fibers are not availablecommercially except as very small volumes. Manufacturing andstorage/transportation concerns require the fiber to besemi-crystalline.

Therefore, it is desirable to provide a polylactide fiber which iseasily manufactured, stored and transported and which degrades rapidlyeven at moderately elevated temperatures.

In one aspect, this invention is a polylactide fiber that contains atleast 75% by weight polylactide resin, wherein

(a) the polylactide resin is a blend of (1) 20 to 90% by weight of afirst polylactide in which the ratio of the R-lactic and S-lactic unitsis from 8:92 to 92:8 and (2) from 80 to 10% by weight of a secondpolylactide in which the ratio of the R-lactic and S-lactic units is≧97:3 or ≦3:97, and wherein the R-lactic units and S-lactic unitscombined constitute at least 90% of the weight of the second polylactide

(b) the ratio of the R-lactic units to S-lactic units in the blend isfrom 7:93 to 25:75 or from 75:25 to 93:7; and

(c) the polylactide fiber contains at least 5 Joules of polylactidecrystallites per gram of polylactide resin in the fiber.

The blend of polylactide is surprisingly and unexpectedly able to formsemi-crystalline fiber. The crystallinity that is obtained issignificantly higher than would be expected from the ratio of R- andS-lactide units that are present in the polylactide resin blend as awhole. The second polylactide appears to crystallize during the fibermanufacturing almost as though the first polylactide is not present atall. The crystallinity permits the fibers to be manufactured easily andto transported and stored without special handling.

Even more surprisingly, the polylactide fiber degrades much more rapidlythan conventional semi-crystalline polylactide fibers, and so is verywell-suited for applications such as the agricultural and subterraneanapplications described above, where rapid fiber degradation is wanted.In particular, the fibers degrade rapidly in the presence of water undermoderate temperature conditions (such as from 50 to 80° C. or from 60 to80° C.), and even under the temperature and moisture conditions that areprevalent in agricultural settings.

In another aspect, this invention is a method for treating asubterranean formation, comprising

a) introducing a treatment fluid into the subterranean formation,wherein

-   -   (i) the treatment fluid contains a liquid phase and multiple        polylactide fibers dispersed in the liquid phase, and    -   (ii) the polylactide fibers have a denier of 0.5 to 20 per        filament and the polylactide resin is a blend of (1) 20 to 90%        by weight of a first polylactide in which the ratio of the        R-lactic and S-lactic units is from 8:92 to 92:8 and (2) from 10        to 80% by weight of a second polylactide in which the ratio of        the R-lactic and S-lactic units is ≧97:3 or ≦3:97, wherein the        R-lactic units and S-lactic units combined constitute at least        90% of the weight of the second polylactide and further wherein        the ratio of the R-lactic units to S-lactic units in the blend        is from 7:93 to 25:75 or from 75:25 to 93:7; and the polylactide        fiber contains at least 5 Joules of polylactide crystallites per        gram of polylactide resin in the fiber, and then

b) degrading the polylactide fibers in the subterranean formation.

-   -   For the purposes of this invention, the terms “polylactide”,        “polylactic acid” and “PLA” are used interchangeably to denote        polymers of lactide having repeating units of the structure        —OC(O)CH(CH₃)— (“lactic units”). The PLA resins each preferably        contain at least 90%, such as at least 95% or at least 98% by        weight of those repeating units.

Either or both of the polylactide resins may contain minor amounts suchas up to 10%, preferably up to 5% and more preferably up to 2% by weightof residues of an initiator compound and/or repeating units derived fromother monomers that are copolymerizable with lactide. Suitable suchinitiators include, for example, water, alcohols, glycol ethers, andpolyhydroxy compounds of various types (such as ethylene glycol,propylene glycol, polyethylene glycol, polypropylene glycol, glycerine,trimethylolpropane, pentaerythritol, hydroxyl-terminated butadienepolymers and the like). Examples of copolymerizable monomers includeglycolic acid, hydroxybutyric acid, other hydroxyacids and theirrespective dianhydride dimers; alkylene oxides (including ethyleneoxide, propylene oxide, butylene oxide, tetramethylene oxide, and thelike); cyclic lactones; or cyclic carbonates. The polylactide resins aremost preferably essentially devoid of such repeating units derived fromother monomers.

The polylactide resin may be capped with a capping agent such as anepoxide, a carbodiimide or oxazoline compound, to reduce and/or increasethe amount of carboxyl terminal groups. Similarly, the polylactide resinmay be reacted with a compound such as a carboxylic anhydride, again toincrease the amount of carboxyl terminal groups. Increasing the amountof carboxyl terminal groups can increase degradation rates; thereforethe amount of these capping agents can be used in some cases to tailorthe degradation rate to a desired value.

Lactic acid exists in two enantiomeric forms, the so-called “S-” (or“L-”) and “R”- (or “D-”) forms. “Lactide” is a cyclic diester made fromtwo lactic acid molecules (with loss of two molecules of water). Thechirality of the lactic acid is preserved when lactic acid is formedinto lactide. Therefore, lactide exists in several forms:3S,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (S,S-lactide),3R,6R-3,6-dimethyl-1,4-dioxane-2,5-dione (R,R-lactide), or3R,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (R,S-lactide or meso-lactide).When lactide is polymerized to form PLA, the chirality is againpreserved, and the PLA so produced will contain S- and R-lactic units inproportions close to the proportion of S- and R-units in the lactide (asmall amount of racemization often occurs during the polymerization).

The fibers of this invention contain a mixture of a first polylactideand a second polylactide. The first polylactide contains S- and R-lacticunits in the ratio of 8:92 to 92:8. That is, at least 8% of the lacticunits in the first polylactide are S-units, and at least 8% of thelactic units in the first polylactide are R-units. It is preferred thatthe ratio of S- and R-units in the first polylactide is from 10:90 to90:10. A more preferred first polylactide contains S- and R-lactic unitsin a ratio of 10 to 50% of S- or R-units and from 10 to 50% of the otherunits. The first polylactide is a highly amorphous grade that iscrystallizable with difficulty and then only to a small extent.Preferably, it is crystallizable to the extent of no more than 5 J/g ofPLA crystallites when quiescently heated (i.e., under no applied strain)at 125° C. for one hour.

At least 97% of the lactic units in the second polylactide are eitherS-lactic units or R-units, i.e., the ratio of S- and R-units is ≧97:3 or≦3:97. This ratio may be ≧98:2 or ≦2:98, ≧98.5:1.5 or ≦1.5:98.5, and maybe as high as 100:0 or as low as 0:100. The second polylactide is asemicrystalline grade that by itself crystallizes easily whenquiescently heated at 125° C. for one hour to produce a semi-crystallinepolymer containing 25 J/g or more of PLA crystallites.

The weight average molecular weights of each of the first and secondpolylactides is suitably within the range of about 30,000 to 500,000g/mol, as measured by gel permeation chromatography against apolystyrene standard.

The first polylactide resin, and preferably both the first and secondpolylactide resin, preferably contains at least some carboxylic acid endgroups. It is more preferred that the blend of polylactide resinscontains about 15 to 50, more preferably from 20 to 30 milliequivalentsof carboxyl end groups per kilogram of polylactide resins.

The polylactide resins can be prepared by polymerizing lactide in thepresence of a polymerization catalyst as described in U.S. Pat. Nos.5,247,059, 5,258,488 and 5,274,073. The polylactide may be a polymer ofany of the lactide types mentioned above, including meso lactide. Thepreferred polymerization process typically includes a devolatilizationstep during which the free lactide content of the polymer is reduced,preferably to less than 1% by weight, more preferably less than 0.5% byweight and especially less than 0.2% by weight. The polymerizationcatalyst is preferably deactivated or removed from the starting high-Dand high-L PLA resins.

The ratio of the first and the second polylactide resins in the fiber isfrom 20 to 90% of the first polylactide resin and correspondingly from80 to 10% of the second polylactide resin, based on total polylactideresin weight. In some embodiments this ratio may be from 30 to 90% ofthe first polylactide and correspondingly from 10 to 70% of the secondpolylactide.

The blend of polylactide resins as a whole may contain a ratio ofR-lactic units to D-lactic units in the range of 7:93 to 25:75 or from75:25 to 93:7. A preferred ratio is from 8:92 to 20:80 or from 80:20 to92:8. A still more preferred range is from 8:92 to 15:85 or from 85:15to 92:8.

The polylactide resins are present as a blend, not as separatecomponents of a multicomponent polymer. The resins may be melt-blendedand/or solution-blended. Melt blending can be performed by separatelymelting the resins, and bringing the melts together, or by forming amixture of particles of the resins and melting the mixture of theparticles together to form the blend. Either of these melt-blendingsteps can be performed as part of the fiber-spinning process, in whichthe melt blend is formed and then spun into fibers without intermediatecooling to form a solid. Alternatively, the melt-blending step can beperformed separately to form a solid mixture of the resins, which isthen re-melted to be spun into the fibers. Similarly, solution blendingcan be performed by separately dissolving the resins and the mixing thesolutions, or by dissolving both resins together. The solution-formingstep can be incorporated into the fiber-spinning process.

The polylactide resins constitute at least 75% of the weight of thefiber, and may constitute as much as 100% thereof. In some particularembodiments, the polylactide may constitute at least 80%, at least 85%,at least 90% or at least 95% of the weight of the fiber. In addition tothe polylactide resin, the fiber may contain, for example, colorants,slip agents, various types of fiber finishes, crystallization nucleatingagents including particulate solids such as talc particles, otherpolymeric materials such as other aliphatic polyesters, polyolefins,poly(alkylene glycol)s and the like and plasticizers. The fiber maycontain one or more agents that increases the hydrophilicity of thepolylactide such as, for example diethylene glycol, triethylene glycol,poly(ethylene glycol). The fiber may also contain one or more catalystsfor the hydrolysis of the polylactide resin, such as a carboxylic acidlike lactic acid, glycolic acid and the like.

The polylactide fiber contains at least 5 Joules, preferably at least 10Joules polylactide crystallites per gram of polylactide resin in thefiber. Polylactide crystallites have a crystalline melting temperatureof from about 140 to 190° C., as measured by differential scanningcalorimetry (DSC). A weighed amount of the polylactide fiber is placedin the differential scanning calorimeter and heated under an inertatmosphere such as nitrogen from room temperature to 250° C. at a rateof 20° C./minute. The enthalpy of melting over the temperature range 140to 190° C. is measured as the amount of polylactide crystallinity in thefibers. This enthalpy is then divided by the weight of the sample todetermine the amount of polylactide crystallites per gram of fiber inunits of Joules/gram.

The polylactide fiber may contain as much as about 30 J/g of polylactidecrystallites. A preferred amount is from polylactide crystallinity isfrom 10 to 25 J/g, and a more preferred amount is from 12 to 22 J/g.These amounts of crystallinity allow the fibers to be processed easilyand provide the fibers with good storage stability.

The polylactide fiber preferably contains no more than 5 J/g, still morepreferably no more than 2 J/g of other crystallites that melt in thetemperature range from 20 to 250° C., as determined by DSC.

Applicants have found that through selection of (1) the ratios of thefirst and second polylactides and (2) the selection of the molecularweight of at least the first polylactide, it is possible to vary therate at which the fiber degrades. This allows a certain amount oftailoring of the degradation rate of the fibers for specificapplications. Within the aforementioned ranges of crystallinity, lowercrystallinity levels tend to promote faster degradation. In addition,fibers in which the first polylactide (or both the first and secondpolylactides) have lower molecular weights also tend to degrade morerapidly. Thus, for example, when faster degradation rates are wanted,the first polylactide may have a weight average molecular weight in therange of 20,000 to 175,000 or from 40,000 to 125,000 or even from 50,000to 100,000. Molecular weights lower than 40,000, especially those below20,000, tend to make it difficult to process the resins into fiber.Conversely, when a somewhat slower degradation rate is wanted, theweight average molecular weight of the first polylactide resin may befrom 100,000 to 300,000, preferably from 125,000 to 250,000.

A suitable test for degradation involves immersing 0.48 g of the fibersin 100 mL of a 0.1 M phosphate buffer solution for 6 days at 65° C. Themass loss of the sample is then determined. In some embodiments, themass loss on this test is 5 to 35%, with values of 7 to 20% on thistest, especially 10 to 20% on this test being preferred.

The fibers may be monofilament fibers, multifilament fibers, and/orconjugate fibers of various types. The fibers can be solid or hollow,and can have any cross-section, including circular, polygonal,elliptical, multilobal, and the like. The fibers can be formed usingsolution-spinning methods, melt-spinning methods, melt-blowing methodsor spun-bonding methods, such as are described, for example in U.S. Pat.No. 6,506,873.

Crystallinity is not an inherent property of the polylactide fibers. Theas-spun fibers typically contain very little crystallinity. Therefore,the fibers in most cases are subjected to some further treatment stepduring which the polylactide resin crystallizes. Such treatment stepsmay include drawing step, in which the fiber is drawn to reduce itsdiameter, and/or a heat-setting step, in which the fibers are heated toa temperature between the glass transition temperatures of thepolylactide resins and their crystallization melting temperatures (suchas between 90 to 140° C.). Drawing can be done in various ways, such asby mechanically stretching the conjugate fiber as it is spun orafterwards, or using a melt-blowing method or spun-bonding method, suchas are described in U.S. Pat. Nos. 5,290,626 and 6,506,873. A heatingstep may be performed by bringing the fiber to a temperature of from 90to 140° C., preferably from 110 to 130° C., for several seconds toseveral minutes. The fibers may be both drawn and heat-set. Drawing andheating steps may be performed simultaneously or sequentially.

The physical dimensions of the fibers are chosen in connection with theintended end-use application. The diameter of the fibers of courseaffects their degradation rates (as does the cross-sectional shape), assmaller diameter fibers have greater surface areas per unit weight. Thediameter of the fibers also affects their physical and flexuralcharacteristics and so is selected in any particular case in accordancewith the requirements of the particular end-use application. The fibersmay have, for example a denier of 0.5 to 100 (weight in grams per 9000meters length) per filament. A more typical denier per filament is from0.5 to 20, more preferably from 0.5 to 5 and still more preferably from0.8 to 2.5.

The fibers may be continuous filament, short “staple” fiber (which mayhave, for example, lengths from 5 up to 150 mm, preferably from 12 to 50mm), and/or in the form of woven or non-woven materials.

The fibers also preferably exhibit no greater than 50%, more preferablyno greater than 25% shrinkage, especially from 5 to 25% shrinkage whenheated in air at 80° C. for 10 minutes.

For certain agricultural applications, the fibers are preferably formedinto a woven or non-woven fabric. The fabric may have openings or poresthat allow a portion of incident sunlight to pass through, while thefabric blocks (by absorption and/or reflection, for example) a remainingportion of the sunlight. The fabric may, for example, allow from 10 to90% or from 25 to 75% of incumbent sunlight to pass through. Such afabric is useful as a plant cover, to protect young plants and/or plantswith tender shoots from being exposed to too much sunlight. Such afabric may also help reduce the loss of moisture from the soil and/orthe plants through evaporation. The fibers typically degrade enoughduring a growing season that they become brittle. The brittle fibers areeasily broken up during tilling, during which they are easily turnedinto the soil where further degradation can occur through the action ofmicrobes.

Certain fibers of the invention are useful in treating subterraneanformations. In such a treatment process, a treatment fluid containingthe fibers is introduced into the subterranean formation, and the fibersare then degraded. In this case, the fibers preferably are in the formof relatively short monofilament and/or multifilament fibers, including“staple” fibers described above.

The treatment fluid includes at least the fibers and a liquid phase. Theliquid phase may include, for example, water, brine, oil, viscous waxesor mixture of two or more thereof. The liquid phase may further containvarious liquid or dissolved functional materials such as thickeners(such as dissolved organic polymers), surfactants, suspension aids, pHadjusters (including acidic or basic materials), pH buffers, and thelike.

The treatment fluid may further contain various suspended solids (inaddition to the fibers) as may be useful in the particular treatmentmethod.

One class of suspended solids includes a proppant. A “proppant” is aparticulate material, insoluble in the treatment fluid, which isintroduced with the treatment fluid in a hydraulic fracturing process tohold open fissures that are produced during the fracturing step.Examples of suitable proppant materials include, for example, sand,gravel, metals, walnut shells, ground coconut shells, various ceramicproppants as are commercially available from CarboCeramics, Inc.,Irving, Tex., and the like.

Another class of suspended solids includes a hydraulic cement, by whichis meant a material or mixture of materials that forms a hard hydrate,including Portland, Portland cement blends, pozzolan-lime cements,slag-lime cements, calcium aluminate cements, calcium sulfoaluminatecements and similar cements. A hydraulic cement may also includeparticles of one or more acid-soluble carbonates.

The treatment fluid may contain, for example, from 1 to 50 volumepercent of the fibers. A preferred amount of fibers is from 1 to 20volume percent.

Well treatment methods of particular interest include hydraulicfracturing and gravel packing.

In a hydraulic fracturing method in accordance with the invention, thetreatment fluid contains the fibers of the invention suspended in theliquid phase. The treatment fluid will in most cases also include aproppant and/or a hydraulic cement. One benefit of the fibers is thatthey help to reduce the settling of the proppant from the liquid phase.The treatment fluid is pumped into the well and into the surroundingformation under high pressure to creates or enlarges fissures in theformation. The fibers, together with any proppant and/or hydrauliccement as may be present, deposit in the fissures. When a proppant orcement is present, the fibers become interspersed within the proppant orcement particles. When the pressure is removed, the deposited materialsprevent the fissures from closing. If a hydraulic cement is present inthe treatment fluid, it will set in the fissures, encapsulating at leastsome of the fibers. When the fibers degrade, they form a liquid orwater-soluble material that is easily washed away either by theproduction fluids or by pumping additional fluids through the formation.The spaces vacated when the fibers degrade form flow paths (wormholes)through the fissures through which the production fluids can flow to thewell bore from which they can be recovered from the well. When aproppant is present, flow paths are formed between the proppantparticles when the fibers degrade. Fibers that are encapsulated incement degrade to form flow paths through the cement. If acid-sensitivecarbonate particles are included in the cement, the acids that form whenthe fibers degrade also helps those particles to dissolve, furtherincreasing the porosity of the cement.

Gravel packing is often performed in a well that extends throughunconsolidated formations that contain loose or incompetent sands thatcan flow into the well bore. In gravel packing, a steel screen, slottedliner, perforated shroud or like device is emplaced in the well tocreate an annulus surrounding the well bore. This annulus is packed withprepared gravel of a specific size designed to prevent sand fromentering the well. In a gravel packing method of this invention, thetreatment fluid includes the liquid phase, fibers as described above,gravel, and optionally a hydraulic cement. The treatment fluid is pumpedinto the annulus such that the fibers, gravel and cement (if present)are captured within the annulus. If a cement is present, it sets in theannulus. As before, flow paths are produced through the gravel pack whenthe fibers degrade.

The fibers are degraded by exposure to water and elevated temperature.Both of these conditions usually exist within wells for producing oil ornatural gas. However, if either or both of these conditions are lacking,or if insufficient water is present and/or a higher temperature isneeded, water and/or heat can be supplied to the subterranean formation.This is conveniently done by injecting steam or hot water into theformation.

The rate at which the fibers degrade will of course depend on thepresence of water and the ambient temperature. Assuming the presence ofenough water, faster degradation is generally seen with increasingtemperature.

An advantage of this invention is that rapid degradation is seen even atmoderate temperatures, such as from 50 to 79° C., especially 60 to 79°C., although higher temperatures, up to 150° C. or greater, can be used.Ambient well temperatures are often adequate. Under these conditions,degradation of the fibers often occurs within 1 to 7 days, moretypically 1-3 days, after emplacement in the formation.

The following examples are provided to illustrate the invention, and arenot intended to limit the scope thereof. All parts and percentages areby weight unless otherwise indicated.

EXAMPLES 1-3

A polylactide resin containing 88% of S-lactic units and 12% of R-lacticunits and having a weight average molecular weight of about 170,000 ispassed multiple times through an extruder. This reduces the molecularweight to about 130,000 g/mol. The so-treated polymer is formed intopellets. 70 parts by weight of these pellets are mixed with 30 parts byweight of pellets of a second polylactide resin that contains 98.6% ofS-lactic units and 1.4% of R-lactic units and has an M_(w) of about160,000. The mixture of polylactide resins contains about 8.8% R-lacticunits and 91.2% S-lactic units. This mixture is melt-spun, heat-set anddrawn to produce multifilament staple fiber having a denier of15-20/filament. The resulting fiber designated as Example 1. It contains15 J/g of polylactide crystallites by DSC, which is very significantlyin excess of the value expected to be obtained with a PLA resin thatcontains a ratio of 8.8:91.2 R- to S-lactic units. The resin processeseasily at high spinning speeds.

Fiber Example 2 is made in the same way, except the ratio of the firstpolylactide resin to the second polylactide resin is 65:35. The mixtureof polylactide resins contains about 8.3% R-lactic units and 91.7%S-lactic units. This fiber contains 9 J/g of polylactide crystallites,which is very significantly in excess of that expected given the ratioof R- to S-lactic units in the resin blend.

Fiber Example 3 is again made in the same way, except the ratio of thefirst polylactide resin to the second polylactide resin is 60:40. Themixture of polylactide resins contains about 7.8% R-lactic units and92.2% S-lactic units. This fiber contains 22 J/g of polylactidecrystallites, which again is unexpectedly high given the ratio of R- toS-lactic units in the resin blend.

Comparative Fiber A is prepared in the same manner, using only the firstpolylactide resin. It contains no measurable crystallinity. These fibersblock when baled and stored at ambient temperatures.

The fiber samples are heated at 57° C. for to assess shrinkage.Shrinkage on this test is a good proxy for the tendency of the fibers toblock (i.e., become stuck together) upon storage at slightly elevatedtemperatures as might be encountered during storage and/ortransportation. Greater shrinkage indicates a greater tendency to block.

Comparative Sample A exhibits 11.5% shrinkage on this test. Examples 1-3exhibit only 8.4, 6.25 and 0% shrinkage, respectively, demonstratingthat the blend of polylactides is much more resistant to blocking atmoderately elevated temperatures than the single resin.

EXAMPLES 4-8 AND COMPARATIVE SAMPLES B-E

The following PLA resins are used to make fiber Examples 4-8 andComparative Samples B-E:

Designation M_(n) M_(w) % R enantiomer A 57,000 111,000 11.7 B60,000-65,000 125,000 50 C 60,500 100,000 1.6 D 66,000 127,000 0.6 E53,500 99,500 4.3

Fibers are spun from PLA resins A-E or blends thereof as indicated inTable 1 below by melt-spinning through a 0.3 mm spinneret, drawing andheat-setting to form circular cross-section solid filaments having adiameter of 12 microns.

The molecular weight of the resins is determined by gel permeationchromatography. The glass transition temperature, melting temperatureand enthalpy of melting are determined on a sample of the resin or resinblend by differential scanning calorimetry. Melting temperature andenthalpy of melting are measured by heating from −25° C. to 225° C. atthe rate of 50° C./minute. Glass transition temperature is measured byheating from 0° C. to 210° C. at 20° C./minute.

Acid end group content is determined by titration.

The fibers are evaluated for blocking by chopping 2.5 g of fiber into2.5-5 cm lengths. The chopped fibers are placed in a preheated cup and apreheated 1 kg weight is applied to the fibers. The assembly is thenplaced in a preheated oven at 80° C. for 10 minutes. The sample is thenremoved and visually inspected to evaluate whether the fibers have stucktogether to form a mass.

To evaluate hot air shrinkage, the fibers are cut into approximately 25cm lengths, measured, and placed on a Teflon sheet. The fibers and sheetare then placed in a preheated 80° C. oven for five minutes. The fibersare then removed and their lengths re-measured.

Hot water degradation is measured as follows: 0.48 grams of 2.5-10 cmfibers are fully immersed in 100 mL of a 0.1M phosphate buffer solution.The container is then heated in a water bath at 65° C. for 6 days. Theflask contents are then filtered through a glass filter and rinsed twicewith 30 mL aliquots of deionized water. The filtered and rinsed fibersare then dried to constant mass in a vacuum oven and weighed todetermine % mass loss.

Results are as indicated in the Table 1.

TABLE 1 Property B* Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 C* D* E* Resin(s) AB/C B/C B/C A/C A/D E C D (50/50) (30/70) (20/80) (70/30) (60/40) % R11.7 24.1 15.0 11.0 8.1 7.0 4.3 1.6 0.6 enantiomer M_(n) 1000 g/mol 5760 66 61 63 64 53 60 66 M_(w) 1000 111 112 109 106 109 117 99 100 127g/mol T_(m), ° C. N/A 172 172 171 168 177 155 174 182 Enthalpy N/A 25.532.8 36.0 19.9 24.0 30.3 45.6 53.1 melting (J/g) T_(g), ° C. 51.7 48.652.6 53.4 55.0 55.5 54.3 56.3 56.0 80° C. Hot Air 70 21.8 13.5 10.3 19.09.8 6.2 6.2 1.5 Shrinkage, % Hot water 25.5 27.8 17.7 12.9 10.8 5.4 5.60 4.2 degradation, % mass loss 80° C. Fail Fail Pass Pass Pass Pass PassPass Pass Blocking

Comparative Sample B shows the effect of using a single, amorphous gradePLA resin that has about 12% of the R-enantiomer. The material cannot becrystallized, and thus has a very high hot air shrinkage value andblocks badly at 80° C. (and lower temperatures).

Comparative Samples C-E show the effect of using a single,semi-crystalline grade of PLA resin that has 0.6 to 4.3% of theR-enantiomer. Shrinkage is very low, but so is degradation, and thesesingle resins are unsuitable for use in applications in which somewhatrapid degradation is necessary.

Examples 4-8 show the effect of using a blend of an amorphous grade ofPLA resin and a semi-crystallizable grade. Example 6 has an overallR-enantiomer content very close to that of Comparative Sample B. Itdegrades more slowly than Comparative Sample B, but does not block andexhibits much less shrinkage on the 80° hot air shrinkage test. Examples4 and 5 show that very high levels of R-enantiomer can be tolerated if ablend of PLA resins is used instead of a single resin (as in ComparativeSample B), and also show how degradation rates can be tailored byadjusting the overall level of the less-predominant enantiomer (theR-enantiomer in this case). As Example 4 shows, degradation rates ashigh as the pure amorphous grade polymer (Comp. Sample B) can beobtained with the blend, while avoiding the very large shrinkage problemexhibited by Comp. Sample B. Example 4 resides at the limits of theinvention, as some tendency to block is seen with this example.

The results in Examples 4-6 are particularly surprising because theamorphous grade of polylactide resin is a polymer of meso-lactide, whichcontains the R- and S-enantiomers in nearly equal amounts and whichcannot be crystallized by itself at all. The use of thepoly(meso-lactide) results in a very high overall R-enantiomer contentin the blend, yet the blend is capable of being crystallized enough toprevent blocking while at the same time providing useful degradationrates.

Example 8 resides at the low limit of overall R-enantiomer content. Thedegradation rate is low for this sample. In this sample, thesemi-crystalline resin has a very low R-enantiomer content. Thatsemi-crystalline resin is believed to crystallize very efficiently (asevidenced by the high crystalline melting temperature for that sample).That efficient crystallization, together with the low overallR-enantiomer level, is believed to account for the low degradation rate.As indicated by the other experiments, a slightly higher overallR-enantiomer content (as in Example 6) is expected to lead to anincrease in degradation rate for that sample.

1. A polylactide fiber that contains at least 75% by weight polylactideresin, wherein (a) the polylactide resin is a blend of (1) 20 to 90% byweight of a first polylactide in which the ratio of the R-lactic andS-lactic units is from 8:92 to 92:8 and (2) from 80 to 10% by weight ofa second polylactide in which the ratio of the R-lactic and S-lacticunits is ≧97:3 or ≦3:97, and wherein the R-lactic units and S-lacticunits combined constitute at least 90% of the weight of the secondpolylactide (b) the ratio of the R-lactic units to S-lactic units in theblend is from 7:93 to 25:75 or from 75:25 to 93:7; and (c) thepolylactide fiber contains at least 5 Joules of polylactide crystallitesper gram of polylactide resin in the fiber.
 2. The fiber of claim 1,wherein the blend of polylactide resins contains about 15 to 50milliequivalents of carboxyl end groups per kilogram of polylactideresins.
 3. The fiber of claim 1, wherein the first polylactide resin hasa weight average molecular weight from 40,000 to 125,000.
 4. The fiberof claim 3, which contains at least 10 Joules of polylactidecrystallites per gram of polylactide resin in the fiber.
 5. The fiber ofclaim 4, which contains 12 to 22 Joules of polylactide crystallites pergram of polylactide resin in the fiber.
 6. The fiber of claim 1, whichloses 7 to 20% of its mass upon immersing 0.48 g of the fiber in 100 mLof a 0.1 M phosphate buffer solution for 6 days at 65° C.
 7. The fiberof claim 6, which exhibits 5 to 25% shrinkage when heated in air at 80°C. for 10 minutes.
 8. The fiber of claim 1, which contains an agent thatincreases the hydrophilicity of the polylactide resin, or a catalyst forthe hydrolysis of the polylactide resin, or both.
 9. A plant coveringcomprising the fiber of claim
 1. 10-17. (canceled)